Many types of air and gas mass flow sensors are presently available for use in connection with internal combustion engines. In most cases, the air intake passage is generally constituted by two flow passages, a main flow passage and an auxiliary flow passage. Often times a hot wire sensor is arranged in the auxiliary flow passage for sensing the flow of air through the passage. One type of mass flow meter including an auxiliary passage range parallel to the main passage is known as a bypass type sensor. A well known bypass type air sensor is disclosed in U.S. Pat. No. 4,723,443 in which the auxiliary passage has first and second flow passages connected transverse to one another. The first flow passage is formed in parallel with the main passage that includes the hot wire sensor disposed within the first flow passage with the second flow passage being arranged perpendicular to the main flow passage thereby returning the flow of air to the main passage. In such an air flow device, the throttle body thereof is divided to form the second flow passage of the auxiliary passage and an isolator is interposed between the divided parts. The second flow passage of the auxiliary passage has a shoulder portion at a location where the second flow passage communicates with the first flow passage. This shoulder portion includes an enlarged width which results in a dead volume of air flow in the auxiliary passage which often times results in inaccurate mass flow measurements.
Another typically known bypass type air flow sensing device is illustrated in U.S. Pat. No. 4,501,249 which includes a second flow passage of an auxiliary passage being arranged radiantly with respect to the main flow passage. Such a device would result in inaccurate mass flow readings in that the air flow cannot be readily conditioned to simulate the mass flow through the main passage.
In an effort to overcome the aforementioned shortcomings, U.S. Pat. No. 4,790,178 and assigned to Hitachi, Ltd. discloses an intake structure for an internal combustion engine including a bypass intake passage for measuring the flow of air to the internal combustion engine. Therein, the intake structure includes a throttle body and an intake body connected to the upstream end of the throttle body which is provided with a bypass air passage for mounting an air flow meter therein. The bypass flow passage portion including the flow meter extends substantially parallel to the intake air passage, however, only a single limited inlet is provided adjacent the intake air passage which may readily result in an uneven and non-representative flow of air through the bypass passage thus resulting in an accurate air flow reading. Similar to U.S. Pat. No. 4,723,443, the return passage extends substantially perpendicular to the bypass passage which again may result in inaccurate mass flow readings.
In yet another effort to overcome the above noted shortcomings, the air flow meter set forth in U.S. Pat. No. 5,119,672 assigned to Robert Bosch GMBH includes an extended bypass passage in an effort to develop a more accurate mass flow measurement. While the extended bypass passage is shifted in the flow direction upstream of the deflection of the air flow into a region of quasi-laminar flow, as with the previous devices utilizing a limited inlet for the bypass flow passage may result in an inaccurate flow measurement for the reasons discussed herein above.
In yet another attempt to ascertain an accurate air mass flow measurement, U.S. Pat. No. 5,186,044 issued to Iragaci et al. discloses an air flow rate measuring device for an internal combustion engine including an auxiliary air passage positioned in a portion of the intake air passage for measuring the air mass flow of the intake passage. However, again such a device obstructs a portion of the intake air flow passage and provides only a inlet in a limited region of the intake air passage for the auxiliary passage which as discussed herein above may result in inaccurate mass flow measurements.
Clearly, there is a need for a mass flow measurement device for measuring air mass flow and gas mass flow to an internal combustion engine. Particularly, there is a need for a device which is able to measure large and small flows with the same basic sensor for use with various engine capacities, one which is insensitive to upstream flow conditions and the particular passage geometry and one which is repeatable from sensor to sensor and from engine to engine to allow interchangability in the field. Moreover, there is a need for a unitary sensing device which senses both the air flow to the internal combustion engine as well as the fuel flow to the internal combustion engine and which provides for a mixing of the fuel with the air being directed to the internal combustion engine.